Imgsizer is a utility that rewrites WWW pages so that image-inclusion tags get the right image size parameters automatically plugged in (this speeds up page loading on many browsers). You can find sources and documentation in the URL WWW tools subdirectory of the ibiblio archive.

imgsizer was originally written in Perl, and was a nearly ideal example of the sort of small, pattern-driven text-processing tool at which Perl excels. It was later translated to Python to take advantage of Python's library support for HTTP fetching; this eliminated a dependency on an external page-fetching utility. Observe the use of file(1) and ImageMagick identify(1) as specialist tools for extracting the pixel sizes of images.

The dynamic string handling and sophisticated regular-expression matching required would have made imgsizer quite painful to write in C or C++; that version would also have been much larger and harder to read. Java would have solved the implicit memory-management problem, but is hardly more expressive than C or C++ at text pattern matching.

In Chapter11 we examined the fetchmail/fetchmailconf pair as an example of one way to separate implementation from interface. Python's strengths are well illustrated by fetchmailconf.

fetchmailconf uses the Tk toolkit to implement a multi-panel GUI configuration editor (Python bindings also exist for GTK+ and other toolkits, but Tk bindings ship with every Python interpreter).

In expert mode, the GUI supports editing of about sixty attributes divided among three panel levels. Attribute widgets include a mix of checkboxes, radio buttons, text fields, and scrolling listboxes. Despite this complexity, the first fully-functional version of the configurator took me less than a week to design and code, counting the four days it took to learn Python and Tk.

Python excels at rapid prototyping of GUI interfaces, and (as fetchmailconf illustrates) such prototypes are often deliverable. Perl and Tcl have similar strengths in this area (including the Tk toolkit, which was written for Tcl) but are hard to control at the complexity level (approximately 1400 lines) of fetchmailconf. Emacs Lisp is not suited for GUI programming. Choosing Java would have increased the complexity overhead of this programming task without delivering significant benefits for this nonspeed-intensive application.

PIL, the Python Imaging Library, supports the manipulation of bitmap graphics. It supports many popular formats, including PNG, JPEG, BMP, TIFF, PPM, XBM, and GIF. Python programs can use it to convert and transform images; supported transformations include cropping, rotation, scaling, and shearing. Pixel editing, image convolution, and color-space conversions are also supported. The PIL distribution includes Python programs that make these library facilities available from the command line. Thus PIL can be used either for batch-mode image transformation or as a strong toolkit over which to implement program-driven image processing of bitmaps.

The implementation of PIL illustrates the way Python can be readily augmented with loadable object-code extensions to the Python interpreter. The library core, implementing fundamental operations on bitmap objects, is written in C for speed. The upper levels and sequencing logic are in Python, slower but much easier to read and modify and extend.

The analogous toolkit would be difficult or impossible to write in Emacs Lisp or shell, which don't have or don't document a C extension interface at all. Tcl has a good C extension facility, but PIL would be an uncomfortably large project in Tcl. Perl has such facilities (Perl XS), but they are ad-hoc, poorly documented, complex, and unstable by comparison to Python's and use of them is rare. Java's Native Method Interface appears to provide a facility roughly comparable to Python's; PIL would probably have made a reasonable Java project.

The PIL code and documentation is available at the project website.